The license for a nuclear steam generating system stipulates the maximum allowable thermal power that the system may produce. The actual measured power that it produces must be less than the allowable power by an amount equal to the uncertainty in the power measurement. Therefore, power measurement accuracy is of continuing importance to the nuclear-power industry, and flow-measurement accuracy, the limiting factor in power measurement, is critically important. As a result of this importance, various methods of measuring the flow have been tried, with particular emphasis on ultrasonic methods. Typically, extremely high-frequency sound waves are sent into the fluid, and the echoes that return from the fluid give an indication of the velocity of the fluid. If the velocity distribution across the cross section of fluid flow is known, the volume rate of coolant flow can be determined.
One of the problems that must be addressed at the outset in designing an ultrasonic fluid-flow measuring system is that of how to propagate the ultrasonic waves into the medium and detect them when they return. The straightforward method of doing this is to place an ultrasonic transducer in contact with the fluid. This means that the transducer is placed within the pressure boundary of the fluid conduit. This can result in an efficient transmission of ultrasonic energy into the fluid and a reception of echoes with a minimum of attenuation. However, ultrasonic systems in which the transducers penetrate to points within the pressure boundary are not preferred.
A way to avoid penetrating the pressure boundary is simply to propagate the waves through the metal that makes up the coolant conduit. The main problem with this method is that it is difficult to get a good transmission of energy across the steel-coolant boundary. If a longitudinal wave is aimed in a direction normal to the interface, the difference in sonic impedances between the two media will cause a reflection of the bulk of the energy back into the steel, ultimately causing a sequence of pulses in response to the original transducer pulse. Only a small part of the energy of each pulse is transmitted into the coolant, and only a small part of the reflection to be detected is transmitted back into the steel from the coolant. Accordingly, a straightforward propagation of the ultrasound directly through the pipe and into the coolant is not very efficient.
One way of avoiding the inefficient transmission of energy across the interface is to transmit shear waves, rather than longitudinal waves, to the interface. If the shear waves impinge on the interface at an appropriate angle, a relatively efficient transmission of energy into the fluid is obtained. However, this assumes that the transducer can effectively propagate shear-mode waves from a transducer into the steel. Unfortunately, in order to do this, it is necessary to have a very rigid bonding between the transducer and the steel. The result of this is that the transducer is subjected to thermally induced stresses as the bond is heated and cooled during operation of the plant. Accordingly, the use of a transducer to transmit shear-mode waves into the steel is not very practical, because either the bond or the transducer will tend to deteriorate.
An indirect method of propagating the ultrasound through the pipe and into the coolant that appears to avoid these problems is the placing of a plastic wedge between the transducer and the coolant pipe. The transducer propagates longitudinal waves through the plastic, and the ultrasound impinges upon the plastic-steel interface at such an angle that a mode conversion occurs, and at least some of the energy is transmitted into the steel as shear-mode waves. The resultant shear-mode waves can be obliquely transmitted across the steel-coolant interface, thereby resulting in less loss during transmission and reception across the interface. Though this would seem to be the ideal arrangement, it also has its drawbacks. The plastic is an extremely lossy medium, so the attenuation that is avoided at the interfaces is encountered in traveling through the plastic wedge.